Display devices, such as liquid crystal display devices, organic electroluminescence display devices and plasma display devices, are often designed to use an image compression technique. When a display panel driver driving a display panel (such as a liquid crystal display panel, an organic electroluminescence display device, and a plasma display panel) incorporates an image memory, for example, use of image compression allows reducing the capacity of the image memory. When image data are compressed before being transmitted to a display panel driver, on the other hand, this effectively reduces the data transmission amount. The reduction of the data transmission amount contributes the reduction of the data transmission rate and power consumption necessary for transmitting the image data.
One of the generally-used image compression methods is the block coding, in which image data are compressed in units of blocks, each including a plurality of pixels. In block coding, image data associated with a plurality of pixels constituting a block are represented by at least one representative value. One major example of block coding is BTC (block truncation coding). BTC is disclosed in, for example, Japanese Patent Application Publication Nos. 2006-42194 A, 2009-212789 A, 2011-19204 A and Japanese Patent Gazette No. 3,790,728 B. The BTC does not provide high data compression ratio and therefore a technique which repeatedly performs the same BCT-based compression procedure a plurality of times has been proposed (see Japanese Patent Application Publication No. 2007-43577 A and Japanese Patent Gazette No. 3,036,058 B.
Another known compression method is a method in which image data of respective pixels are transformed into data of the frequency domain. One major example of such compression method is DCT (Discrete Cosine Transform). The DCT is a basic technique of image compression such as JPEG (Joint Photographic Experts Group) and widely used in image processing.
When the compression processing is performed with a fixed data length (that is, when the compression ratio is fixed), however, both of the BTC and DCT cause a large compression error for specific types of images, and this results in image deterioration. More specifically, the BTC causes severe image deterioration for images including a large amount of low frequency components (for example, images of photographs and the like), while the DCT causes severe image deterioration for images including a large amount of high frequency components (for example, images including characters and the like). A display device of a portable terminal (for example, smart phones and the like) is often required to display mixture images of characters, still pictures and graphic pictures (for example, when displaying a web site), and under such situations, it is not preferable to use image processing which suffers from a severe image deterioration for a specific type of image.
In order to address such problem, the inventors have proposed a technique in which a compression method is selected in response to the correlation among pixels within the block (see, Japanese Patent Gazette No. 4,507,265 B, for example). One feature of this technique is that a correlation is calculated for every possible combination of two pixels selected from the pixels included in the block. The selection of the compression method in response to the correlations calculated for all the possible combinations effectively reduces the compression error and suppresses the image deterioration.
Although being effective for reducing the compression error, the technique of selecting the compression method in response to the correlations among the pixels in the block undesirably increases the circuit size when pixels to be processed at the same time (that is, the number of pixels included in each block) is increased, due to the increase in the number of combinations of the pixels. In the case that the number of pixels included in a block is four, for example, the number of possible combinations of two pixels from the block is six (=4C2). In the case that the number of pixels included in a block is eight, on the other hand, the number of possible combinations of two pixels from the block is 28 (=8C2).
Therefore, a technical need exists for providing a technique for reducing the compression error with a reduced circuit size.